1. Field of the Invention
The present invention relates to a method for ultra-high speed optical transmission making use of quasi-solitons in fibers.
2. Description of the Related Art
The following is literature related to studies on optical soliton communications.
(1) M. Suziki, I. Morita, S. Yamamoto, N. Edagawa, H. Taga and S. Akiba, Optical Fiber Communications (OFC'95), Opt. Soc. Am., Washington D.C., Paper PD20 (1995).
(2) H. A. Haus, K. Tamura, L. E. Nelson and E. P. Ippen, IEEE J. Quantum Election. QE-31, 591 (1995).
(3) F. M. Knox, W. Forysiak and N. J. Doran, J. Lightwave Technol. LT-13 1955 (1995).
(4) N. J. Smith, F. M. Knox, N. J. Doran, K. J. Blow and I. Bennion, Electron. Lett. 32, 54 (1996).
This literature reports formation of a stable soliton-like Gaussian pulse through an increase in power.
(5) T. Georges and B. Charbonnier, "Reduction of the dispersive wave in periodically amplified links with initially chirped solitons", submitted to Photon. Techn. Lett.
According to this literature, when pre-chirping is applied to soliton input to fibers composed of positive dispersion fibers and negative dispersion fibers connected alternately, generation of dispersive wave and interaction between solitons are suppressed.
(6) J. D. Moores, Opt. Lett. 21, 555 (1996).
(7) P. A. Belanger and N. Belanger, Opt. Comm. 117, 56 (1995).
According to this literature, when a pulse is subjected to chirping, the pulse contracts.
As described above, extensive studies have been conducted in relation to optical soliton communications, which have excellent characteristics, such as the capability of forming stable and stationary localized pulses. In conventional soliton communications, group velocity dispersion of pulses in the fibers is compensated by making use of the non-linear effect caused by the Kerr effect.
FIG. 1 shows a conventional soliton transmission scheme.
As shown in FIG. 1, light from a light source 13 is modulated by modulation means 12 in accordance with an electronic signal 11, so that solitons are generated from the modulation means 12. After being amplified by an optical amplifier OA.sub.1, the solitons are transmitted through optical fibers OF.sub.a-1, OF.sub.a-2, OF.sub.a-3, and OF.sub.a-4 having varying dispersions and constituting a first fiber section. The solitons are then transmitted successively through second and third fiber sections, and enter an optical amplifier OA.sub.4 in which the solitons are amplified. Subsequently, the solitons are transmitted through optical fibers OF.sub.d-1, OF.sub.d-2, OF.sub.d-3, and OF.sub.d-4 having varying dispersions and constituting a fourth fiber section. The solitons are then passed through an optical amplifier OA.sub.5 and are received by a light receiver 16. The received solitons are demodulated by demodulation means 17 so as to yield an electronic signal 18 corresponding to the original electronic signal 11.
However, as shown in FIG. 2, in the above-described conventional optical soliton communications scheme, each pulse has a shape defined by a hyperbolic function and has a long skirt, resulting in a problem of proneness to interaction with an adjacent pulses.
The conventional scheme has another problem that when pulse width is decreased to 10 picoseconds or less, peak power exceeds 10 mW.